Rather than interrupting a task, I could shut down my computer when everything is idle in order to perform updates and other maintenance.

Tasks write to checkpoint files (at least here on SETI and most other BOINC projects) and can resume where it left off after a shutdown. I do this all the time to clean out my systems of all the dust.

I understand, I'm just anal.

With most projects you lose more CPU time by not being present when the task ends to turn off the machine than by just losing the average 30 seconds (with the default settings) that you lose when BOINC shuts down gracefully in the middle of working.BOINC WIKI